The present invention relates generally to systems and methods for increasing the efficiency of a field oriented induction motor of a given design and, more specifically, to systems and methods for selecting the associated torque current and flux current such that the motor may operate at a peak torque per amp (PTPA) point.
The xe2x80x9cfuelxe2x80x9d powering a field oriented induction motor is current. This current may be divided into two components, torque current and flux current. The torque current may be viewed as that component of the current which generates motive force, or torque. The flux current may be viewed as that component of the current which generates magnetic flux in the rotor. Shaft torque and rotor flux are related, with shaft torque proportional to the product of rotor flux times torque current.
The torque current and flux current associated with a field oriented induction motor may be continually adjusted during motor operation. Relative to the present invention, the ratio between the torque current and flux current is also of importance in motor design. Motors of the same design, incorporating, for example, the same amount of magnetizable and conductive material, and the same lamination characteristics, typically have the same ratio. The ratio between the torque current and flux current has significant implications for motor efficiency.
Typically, it is assumed that all motor parameters are constant across a given operating range, and mathematical models predict that for optimum efficiency the torque current and flux current should be made equal. Alternatively, a predetermined ratio may be assumed to exist between the torque current and flux current. This ratio may be an approximation based upon experimentation, for example. These assumptions and approximations are limited, however, by the fact that for a field oriented induction motor there is ultimately a maximum flux which may be achieved. This is due to what is known in the art as the xe2x80x9csaturation effect.xe2x80x9d At some point, increasing the flux current to a field orientation induction motor results in only a marginal increase in flux. This diminishing return is a result of the physical characteristics, and limitations, of the materials which make up the motor. Due to the saturation effect, it is desirable, once saturation has occurred, to divert current to produce torque, rather than flux. Thus, due to the saturation effect, the traditional assumptions and approximations, such as those discussed above, often lead to undesirable inefficiencies.
The present invention overcomes the problems discussed above and provides systems and methods for increasing the efficiency of a field oriented induction motor. Specifically, the present invention provides systems and methods which allow the current used by a field orientation induction motor to be minimized, while the torque produced by the motor is maximized, allowing the motor to operate at the peak torque per amp (PTPA) point. This is accomplished by providing a motor designer with the tools necessary to determine the optimum ratio between torque current and flux current.
In one embodiment, a field oriented induction motor system includes a field oriented induction motor having an associated torque current and an associated flux current and a predetermined current ratio, wherein the predetermined current ratio is defined as the ratio of the torque current to the flux current, and wherein the predetermined current ratio is dependent upon the saturation state of the motor.
In another embodiment, a method for selecting the ratio of torque current to flux current for a field oriented induction motor includes applying an allocation factor to the torque current and flux current, wherein the allocation factor is dependent upon the saturation state of the motor.